Bi-directional communications may use communication links or channels through which signals are delivered. Such signals may be sent over wires, optical fibers, or be modulated for delivery through space in the form of RF communications or for delivery through water or air in the form of acoustic communications. Bi-directional communications address interference problems. For example, when two nodes exchange communications, each node must be able to distinguish its own transmitted signals from the signals transmitted by the other node.
Conventional bi-directional communications contemplate full duplex or half duplex operation. Full duplex operation uses two, oppositely directed, simplex or unidirectional links which operate simultaneously. No interference results because the two links operate independently from one another. For example, one link may use an entirely separate set of wires or fibers from the other link, or one link may use an entirely separate frequency band or set of frequencies from the other link. With full duplex operation, each party transmits over one link while simultaneously receiving over the other link. Half duplex operation utilizes only a single link. Communication may travel in either direction over this single link so long as it travels in only one direction at a time. Interference is avoided because nodes which utilize the link refrain from transmitting and receiving at the same time.
Full duplex operation is often preferred over half duplex operation. Assuming that a single half duplex link and the two simplex links used by a full duplex operation have the same capacity, a greater amount of data may be communicated using full duplex operation. Moreover, with full duplex operation, link management and timing concerns become trivial. On the other hand, with half duplex operation the communication system must devise schemes to determine when to allow different nodes to transmit over the link.
When RF communication links are established through space, and particularly outer space, the desirability of full duplex operation fades. For example, even though two simplex links may utilize different portions of the spectrum, transmission circuits adapted to operate in one portion of the spectrum must be isolated from receive circuits that are adapted to operate in another portion of the spectrum. This isolation is often accomplished at the cost of massive shielding and increased circuit complexity. When the node operating one end of the link is a satellite, the weight of shielding and increased circuit complexity can make satellite costs excessive.
Moreover, when a node operates more than one bi-directional link, the total number of independent spectrum bands needed to carry non-interfering communications increases. Thus, a given amount of spectrum may be divided into a greater number of closely spaced independent frequency bands. As the number of frequency bands increases, so do the isolation problems discussed above.
While conventional half duplex operation does not suffer from the isolation problems associated with full duplex operation, it has its own problems. The half duplex requirement that only one node use the link at a time results in an inefficient use of the link. Nodes may be required to wait between transmitting one communication and receiving another. The wait is imposed by the time it takes the communications to travel between nodes, and is often greater than the duration of a communication signal's round trip between the nodes. For example, a first node that has just completed a first transmission into a link must wait for a second node to finish receiving the first transmission, for the second node to begin a second transmission, and for the second transmission to reach the first node, before receiving data from the link. When nodes are located hundreds or thousands of miles apart, this wait can impose significant delays on communication. Moreover, as more nodes engage in half duplex communication among each other, the amount of data that can be delivered between any two nodes decreases.